Overvoltage protection is typically provided by voltage dependent resistors, such as Schottky diodes based on SiC, or varistors, based on ZnO, which work on solid state principles related to grain boundary conduction.
The most popular type of voltage dependent resistors, or varistors, are based on zinc oxide doped with other elements to control the grain boundaries. These devices depend on their non-linear I-V behavior transient voltage surges. However, there are some significant compromises that result from their use. Voltage permanently applied to the varistor must be carefully limited to avoid excessive power dissipation. Since they often have a negative temperature coefficient of resistivity a runaway condition can easily be precipitated. Subjecting varistors to electric fields can change the characteristic and result in an increase in current and power dissipated as heat degrading performance.
There has been an ongoing desire for an overvoltage protection device which does not have the deficiencies of the prior art.